1. Field of the Invention
The present invention relates to a polarizing plate, a method for manufacturing the same, and a liquid crystal display panel having the same, and more particularly, to a polarizing plate in which bending due to difference between shrinkage and expansion rates is prevented by controlling a moisture absorption rate of a compensation film thereof, a method for manufacturing the same, and a liquid crystal display panel having the same.
2. Description of the Related Art
In general, the application of liquid crystal displays (LCDs) has been extended because of their features of light weight, slimness, low-power drive, full colors, high resolution and the like. At present, LCDs are used in computers, notebook computers, PDAs, telephones, TVs, audio/video devices, and the like. Such an LCD displays desired images on an LCD panel by adjusting the quantity of light to be transmitted in response to image signals applied to a plurality of control switches arrayed in a matrix.
FIG. 1 is a view illustrating a structure of a general polarizing plate. The polarizing plate shown in FIG. 1 comprises a polarizer 21, protection films 22a and 22b, an adhesive 23, and surface protection films 24a and 24b. The polarizer 21 is arranged between the protection films 22a and 22b to polarize incident light. The adhesive 23 has the function of attaching the polarizing plate to a substrate such as an LCD panel. The surface protection films 24a and 24b are attached to protect the polarizing plate from foreign substances and are separated from the polarizing plate when a process of attaching the polarizing plate to the substrate is performed.
FIGS. 2a and 2b are sectional views of a conventional polarizing plate having an optical compensation film and a conventional polarizing plate having an integrated type protection film, respectively.
The polarizing plate shown in FIG. 2A comprises a polarizer 31, protection films 33a and 33b arranged on the top and the bottom of the polarizer 31 to protect the polarizer 31, respectively, and an optical compensation film 35 arranged on one of the protection films 33a and 33b. The polarizer 31 may include poly vinyl alcohol (PVA), and the protection films 33a and 33b may include tryacetyl cellulose (TAC). A viewing angle compensation film having a viewing angle compensation function may be used as the optical compensation film.
The polarizing plate shown in FIG. 2B comprises a polarizer 41, a protection film 43 arranged on one surface of the polarizer 41 to protect the polarizer, and an integrated type protection film 45 arranged on the other surface of the polarizer 41 to have a polarizer-protecting function as well as an optical compensation function. The polarizer 41 may include poly vinyl alcohol (PVA), and the protection film 43 may include tryacetyl cellulose (TAC).
As described above, the optical compensation film 35 or the integrated protection film 45 is different from the protection film in view of materials and components. Thus, there is difference in moisture absorption rate between them. As a result, there is a problem in that the polarizing plate may be bent due to difference between shrinkage and expansion rates of components of the polarizing plate. This will be described in detail below with reference to FIG. 3.
FIGS. 3A and 3B are views showing differences between shrinkage rates of components of a conventional polarizing plate in a state where the polarizing plate with an optical compensation film is attached to an LCD panel.
FIG. 3A shows a state where a polarizing plate with the optical compensation film 35 of FIG. 2A is attached to the top of an LCD panel 39 using an adhesive 37, and FIG. 3B shows a state where a polarizing plate with the integrated protection film 45 of FIG. 2B is attached to the top of an LCD panel 49 using an adhesive 47.
Referring to FIG. 3A, the moisture absorption rates of the polarizer 31, the protection films 33a and 33b, and the optical compensation film 35 are different from one another. Since both moisture absorption and shrinkage rates are generally in substantially proportion to each other, a shrinkage or expansion rate increases as a moisture absorption rate increases. In general, the moisture absorption rate of poly vinyl alcohol (PVA) is about 5% or more, that of tryacetyl cellulose is about 1.5% and that of an optical compensation film is about 3%. Accordingly, in FIG. 3A, dimensional changes become larger in the order of the protection films 33a and 33b (ΔL3), the optical compensation film 35 (ΔL1) and the polarizer 31 (ΔL2).
Even in the case of FIG. 3B, since the moisture absorption rates of the polarizer 41, the protection film 43 and the integrated protection film 45 are different from one another, there is difference between dimensional changes of the components when the polarizing plate is attached to an LCD panel and the LCD panel is then driven.
As described above, the dimensional changes cause bending of the polarizing plate and thus there is a problem in that the polarizing plate gets loose from the LCD panel, which consequently causes a problem of light leakage.
FIGS. 4A to 4C are views showing light leakage defects in driving a conventional LCD panel to which a polarizing plate with an optical compensation film is attached. FIG. 4A shows the state of the LCD panel during initial driving, and FIG. 4B shows a state where a light leakage phenomenon occurs in the LCD panel after driving for 50 hours. FIG. 4C shows a state where a light leakage phenomenon occurs in the vicinity of corners of the LCD panel upon estimation of reliability thereof.